Per the parent ticket, we need to a migration path to avoid breaking rendering of existing translatable pages. In order to do that, we need to:
- track and store syntax version of a translatable page
- allow migration when re-marking the translatable page
- (deferrable) provide a list of unmigrated pages
Affected projects: All Wikimedia projects using Translate and translatewiki.net
Pre/post-deployment QA: Check Special:PageTranslation, upgrade a page, create a new page and mark it. Neither should be using the old syntax. Check that form elements are consistent (if OOUI patch is merged by the time)
New pages will use new syntax version, old pages can be migrated by translation admins when marking a page and ticking a checkbox.